Heretofore display signs, such as exit signs, used incandescent lamps and fluorescent lamps as sources of illumination. While these lamps provide relatively uniform sources of a broad visible spectrum of light, there are several problems that exist when using these lamps in exit sign applications.
Incandescent lamps are relatively inefficient which results in large amounts of entrapped heat energy, shortened life of the bulbs and increased operating costs. The use of incandescent bulbs in exit sign applications requires that suitable ventilation be finished in the design of these signs to remove the entrapped heat. Also, an added undesirable attribute is the frequent maintenance that is required, such as, periodic bulb replacement when lamp failure occurs. Because these lamps are inefficient in operation, there are increased operating costs.
While the use of fluorescent lamps circumvents the problems that are associated with entrapped heat, frequent lamp replacement, and inefficient operation; the size of the fluorescent lamp together with its required ballast, the necessity of higher voltage operation using alternating currents and the higher cost of replacement lamps, offset the advantages for its long term use.
Most recently, semiconductor LED lamps have been used in display and exit sign applications. The use of conventional semiconductor light emitting diodes solves the problems of entrapped heat, lamp longevity, frequent lamp replacement and of higher voltage operation.
However, one problem that exists when using conventional light emitting diodes is that of an non-uniform distribution of emitted light energy. The first attempt in using these diodes as sources of illumination was to provide a diffusion plate to minimize the uneven illumination. While many conventional LED lamps use water clear lenses, still another solution to this problem was to design a conventional LED lamp that included a diffused lens and envelope, such as the Panasonic LN21XP.
Shown in FIG. 1 is a sectional view of a conventional LED lamp having one or more semiconductor chips 150. The transparent envelope 10 has a convex hemispherical shaped lens 12, located at the end of a cylindrically shaped rod 14. A semiconductor chip 150 is mounted to the conductive surface of the first electrode, LED anode 110, which is recessed within the convex reflector 140. A fine wire 130 connects the opposite end of the semiconductor chip 150 to the second electrode 120.
Turning now to FIG. 2, shown is the cone shaped light pattern 16 that is emitted from this LED device. A beam of visible light projects in a cone that ranges in angularity from 12 to 36 degrees dependent upon the intended design usage. This designed lamp finds particular use in instrument panel lamp applications.
Referring now to FIG. 3, illustrated is a sectional view of another conventional LED lamp having one or more semiconductor chips 150. The transparent envelope 20 of the cylindrical shaped rod 14 has a flat top-hat shaped lens 22. The semiconductor chip 150 is mounted to the conductive surface of the first electrode, the LED anode 110, which is recessed within the convex reflector 140. A fine wire 130 connects the opposite end of the semiconductor chip 150 to the second electrode 120.
As shown in FIG. 4, the light that emanates from the semiconductor chips is projected on the flat surface lens 22, thereby giving a uniform surface of illumination that is suitable for use in instrument panel lamp applications.
Examples of the prior art conventional light emitting diode structures will now be discussed in some detail.
U.S. Pat. No. 5,289,082, granted Feb. 22, 1994, S. Komoto, discloses an LED lamp having a lead portion upon which are mounted a plurality of semiconductor chips placed on the tip portion of the lead, all surrounded by a translucent envelope. The envelope is characterized as being formed from a solid body containing parts of a plurality of ellipsoids whose axes extend through the semiconductor chips.
U.S. Pat. No. 5,083,192, granted Jan. 21, 1992, to J. Rzeznik, et al, discloses light concentrating cluster mount for super bright high intensity light emitting diodes.
U.S. Pat. No. 4,849,803, granted Jul. 18, 1989, to T. Yamamoto, et al, discloses a molded resin semiconductor device having a plurality of semiconductor chips surrounded by a low thermal expansion, high thermal conductivity thermosetting resin. This in turn is encapsulated in a flexible resin having high expansion and low conductivity coefficients. A final thermosetting resin coating completes the assembly.
U.S. Pat. No. 4,041,516, granted Aug. 9, 1977, to R. W. Murray, discloses a high intensity light emitting diode having improved radiation intensity and distribution characteristics.
U.S. Pat. No. 4,032,963, granted Jun. 28, 1977, to G. P. Thome, teaches a method and structure for encapsulating a light emitting device. A package is provided to prevent failure due to chemical contaminants.
Many of the above referenced prior art disclose methods and apparatus for concentrating the radiant light energy into a narrow conical beam to optimize the viewing angle so that the light emitting diode is suitable for use in panel lamp applications. Also, some of the above referenced prior art disclose the use of reflective surfaces mounted beneath the array of semiconductor devices to further concentrate the beam emanating from these diodes.
Therefore, there is a particular need for a light emitting diode lamp that provides a source of radiant energy that is uniformly distributed spherically, encompassing all directions so that it can be viewed in any direction, from any circumferential angle, at any angle of elevation.
In this regard, this invention fulfills this need.